1. Field of the Invention
This present invention relates to semiconductor devices, and particularly to a semiconductor device having protection elements for preventing output buffer MOSFETs from electrostatic breakdown.
2. Description of the Related Art
A semiconductor device having a MOS integrated circuit, an output buffer circuit and a voltage supply circuit has heretofore been known. The voltage supply circuit is a circuit which supplies a voltage to a semiconductor substrate to activate the MOS integrated circuit at high speed and in a stable state.
With regard to the above-mentioned semiconductor device, there is known one device which is disclosed in, for example, "IEICE TRANSACTION" VOL. E77-A p. 166-172, 1994.
The disclosed semiconductor device has offset gate MOSFETs (MOSFETs for prevention of electrostatic breakdown) for preventing buffer MOSFETs from electrostatic breakdown. The electrostatic-breakdown preventive MOSFETs and the buffer MOSFETs are electrically connected in parallel between an output terminal of a MOS integrated circuit and a ground terminal.
Even in the case of this type of semiconductor device, however, the buffer MOSFETs cannot be fully prevented from electrostatic breakdown.